FIG. 1 illustrates a radio frame format (using partial usage of sub-channel (PUSC) scheme) of worldwide interoperability for microwave access (WiMAX) standard specifications defined by the IEEE 802.16e, which is a related art.
In the format illustrated in FIG. 1, sub-channels (the number of sub-channels is 60, and the bandwidth is 20 MHz) of an orthogonal frequency division multiplexing access (OFDMA) system are divided into six sub-channel groups #0 to #5, and radio resources are distributed to sectors accommodated by a base station.
For example, as illustrated in FIG. 1, in a case where the base station includes three sectors S1 to S3, the radio resources of the sub-channel groups #0 and #1 are allocated to the sector S1, the radio resources of the groups #2 and #3 are allocated to the sector S2, and the radio resources of the groups #4 and #5 are allocated to the sector S3.
In such a frame format, a structure (layout position) of a terminal broadcast information area such as a frame control header (FCH) or a downlink/uplink-MAP (DL/UL-MAP) is fixed to sub-channel groups (#0, #2, and #4) having even numbers. Accordingly, one radio resource cannot be divided into three or more sectors.
Further, in a case where the communication area of a base station is divided into three sectors, in order to prevent interference, another base station located in the vicinity of the base station cannot be allocated with frequencies in the same band.
Further, as a radio resource allocation method, it is conceivable to employ fixed radio resource allocation in which, with respect to a base station including a plurality of sectors, the radio resources are equally allocated in a fixed manner among all the sectors. For example, as illustrated in FIG. 2, when the radio resources are equally allocated in a fixed manner to sectors 1 to 3, there may occur a case in which, at a certain time point, the number of users accommodated in the sector 1 is small, the number of users accommodated in the sector 2 is large, and the number of users accommodated in the sector 3 is moderate. In this case, such a situation may occur in which the radio resources of the sector 1 have an unused part left whereas the sector 2 runs short of radio resources. As described above, with the fixed radio resource allocation, it is impossible for the base station as a whole to attain a maximum throughput when users (terminals) concentrate on a particular sector among a plurality of sectors.
On the other hand, in dynamic radio resource allocation, a ratio of the radio resources to be allocated to each sector with respect to all the radio resources is dynamically changed in accordance with the number of users accommodated in each sector. In the dynamic radio resource allocation, under the above-mentioned situation, it is possible to reduce the ratio of the radio resources to be allocated to the sector 1 while increasing the ratio of the radio resources to be allocated to the sector 2. With this configuration, even when the number of accommodated users is disproportionately large in a particular sector, it is possible to attain a maximum throughput in the base station as a whole, and hence efficient utilization of frequency bands can be achieved.
FIG. 3 is an explanatory diagram for a case in which a reuse factor of 1 (Reuse Factor=1) is realized by using dynamic channel allocation control as a related art. In the case of the reuse factor of 1 with the use of this dynamic channel allocation control, the same frequency band is used among base stations (sectors), and, in accordance with the use status of the radio resources of each of the base stations, the radio resources are dynamically allocated to the respective base stations (in FIG. 3, base stations S1 to S6) in units of sub-channels. Specifically, the use status of the radio resources is checked (measured) at fixed intervals for each base station, whereby radio resource distribution to each base station (the number of sub-channels to be allocated to each base station (sector)) is dynamically changed.    Patent Document 1: JP 10-126846 A
However, the related art illustrated in FIG. 2 (the case of the Reuse Factor=1 with the use of the dynamic channel allocation control) has the following problems.
<1> The structures inside radio frames need to be made identical among the base stations. This prevents the individual base stations from flexibly operating a technology that utilizes a plurality of antennas, such as multiple-input multiple-output (MIMO) or an adaptive array system (AAS).<2> When the sub-channels are used randomly among a plurality of base stations (sectors), it becomes difficult to take measures against partial fading or the like.<3> The radio resources are divided into small segments, leading to decreased efficiency in operation.